The present invention relates to a coated cutting tool (cemented carbide insert) particularly useful for wet turning of toughness-demanding stainless steel components like square bars, flanges and tubes, with raw surfaces such as cast skin, forged skin, hot or cold rolled skin or pre-machined surfaces.
When turning stainless steels with cemented carbide tools, the cutting edge is worn according to different wear mechanisms, such as adhesive wear, chemical wear, abrasive wear and by edge chipping caused by cracks formed along the cutting edge, the so-called comb cracks.
Different cutting conditions require different properties of the cutting insert. For example, when cutting in steels with raw surface zones, a coated cemented carbide insert must consist of a tough carbide and have very good coating adhesion. When turning in stainless steels, the adhesive wear is generally the dominating wear type.
Measures can be taken to improve the cutting performance with respect to a specific wear type. However, very often such action will have a negative effect on other wear properties.
So far it has been very difficult to improve all tool properties simultaneously. Commercial cemented carbide grades have therefore been optimised with respect to one or few of the wear types and hence to specific application areas.
Swedish patent application 9503056-5 which corresponds to U.S. Ser. No. 08/703,965, herein incorporated by reference, discloses a coated cutting insert particularly useful for turning in hot and cold forged low alloyed steel components. The inserts is characterised by a cemented carbide substrate consisting of Co--WC and cubic carbides having a 15-35 .mu.m thick surface zone depleted from cubic carbides, a coating including a layer of TiC.sub.x N.sub.y O.sub.z with columnar grains, a layer of smooth, fine grained .kappa.-Al.sub.2 O.sub.3, and preferably an outer layer of TiN.
Swedish patent application 09/077,360 9504304-8 which corresponds to U.S. Ser. No. 09/077,360 filed concurrently herewith discloses a coated cutting insert particularly useful for wet and dry milling of low and medium alloyed steels. The insert is characterised by a cemented carbide substrate consisting of Co--WC and cubic carbides, a coating including a layer of TiC.sub.x N.sub.y O.sub.z with columnar grains, a layer of smooth, fine grained .kappa.-Al.sub.2 O.sub.3 and preferably an outer layer of TiN.
It has now been found that combinations of the substrates and coatings described in the above patent applications give rise to excellent cutting performance in stainless steels turning. A cemented carbide substrate with a cubic carbide depleted surface zone combined with a coating in accordance with patent application, 9503056-5, which corresponds to U.S. Ser. No. 08/703,965, has been found to be especially suitable for high speed turning in easy stainless steel, such as turning of machineability improved 304L, In more difficult work piece materials such as 316-Ti, and in operations with a high degree of thermal cycling such as turning of square bars a straight WC--Co substrate of the type described in patent application 09,077,360 9504304-8 which corresponds to U.S. Ser. No. 09/077,360 filed concurrently herewith, has been found the most suitable.